Well hold on. I wasn't referring to trucks or wagons, but to half tracks. Page 43 of my manual (it's real old) under the heading "Assault" says: "To assault, the attacker(s) must have an assault value greater than "0"... and must have enough APs to assault...". Page 44 has a bunch of info pertaining to assault values and specifically mentions armored carriers capable of assaulting.
As for trucks, wagons, motorcycles, rafts, etc. nonoccupied and fog-of-war in effect, it says of page 30:"...Note that horses, motorcycles, bicycles, boats and rafts cannot move or be used for spotting purposes... , whereas trucks, halftracks and artillery tractors (because each is considered to have an "inherent" driver) can move (or be used for spotting purposes) if not loaded. The game enforces the rule by placing a question mark over a spotted unit (pretty stupid, but that's how they designed it).
I'm not sure if I ever used a truck to help surround a unit, but must confess I probably have, not that I'd ever do it in real life, especially if I was the driver.
While we're on the subject, someone here once remarked that they didn't like playing the rule that allows you to exit units off the map to avoid point losses. This is not only allowed, the book actually suggests it as a viable strategy (page 68).
So there you have it. I can't believe I actually looked this stuff up. I better go lie down now.